Total synthesis of calothrixins A and B via oxidative radical reaction of cyclohexenone with aminophenanthridinedione

Bioactive indolo[3,2-j]phenanthridine alkaloids, calothrixin B and its N-oxide derivative calothrixin A have been synthesized via an oxidative free radical reaction. Calothrixin B is generated from the commercially available 2,4,5-trimethoxybenzaldehyde in only seven steps. The key step in this synthesis is the Mn(OAc)3 mediated oxidative free radical reaction of 9-(benzylamino)phenanthridine-7,10-dione with cyclohexenone to form 12-benzyl-12H-indolo[3,2-j]phenanthridine-7,13-dione.